One of the main applications of the invention relates to any occasion when premises are under surveillance for the purpose of triggering some type of alarm on detecting a predetermined intrusion into the premises; other applications include fluid mechanics, and making artificial skins.
Another application is utilization in sport, e.g. whenever there is a need to detect whether a ball strikes on or outside a given line, as in tennis, while avoiding disturbances due to other types of impact, such as those from the feet of the players.
Various detection devices and methods are known that make use of various techniques: some of them are active in that they emit waves e.g. light waves or sound waves or electromagnetic waves, and they detect any mass or object (whether identified or not) moving or taking up a position in the space under surveillance by the waves by virtue of the waves being reflected.
Other devices are passive in nature: they do not emit any wave or signal in the absence of a disturbance and they trigger only in the event of a change of state in a parameter under surveillance (e.g. temperature, pressure, contact, luminosity, noise, etc. . . .).
All of these methods and corresponding apparatuses satisfy a given utilization given the desired detection criteria, and each has its own degree of quality and reliability. In particular, when it is desired, as in the present invention, to detect impacts, use is made essentially either of simple on/off contact systems which trigger at a predetermined threshold of contact displacement, or else of measuring sensors which make it possible to attempt to find out more about variation in a physical parameter such as pressure, so as to have a quantitative measurement of an impact on the basis of which various actions may be triggered.
In this field, mention may be made of a patent application filed Feb. 15, 1978 in the United States under the number 878 056 by "The Bendix Corporation" and describing "An assembly including a capacitive sensor and a demodulation circuit", with the claims thereof bearing essentially on the circuit which is required to compensate for variations in the output signal produced by system errors. It is known that any measurement of the capacitance in a circuit is a function, in particular, both of the temperature and of the humidity of the medium. It is therefore necessary in apparatuses and sensors of this type to make corrections therefor in order to compare pressure measurements under the same conditions.
Various other patent applications have been filed for other types of capacitive sensor, e.g. an application made by S.I.E. GmbH in West Germany on May 1, 1987, under the number G 87 06 280.1, describing a sensor including three flexible electrodes and an electronic circuit delivering a signal which is a function of the deformation of said electrode and due, for example, to variation in the pressure on the sensor.
In order to obtain information not only at a point, but also over an area, various sensors have been developed for detecting deformations therein by using electrical measurements. French patent application No. 75/34.225 filed Nov. 5, 1975 by the French national agency for promoting research "AGENCE NATIONALE DE VALORISATION POUR LA RECHERCE A.N.V.A.R.", describes a touch sensor for the purpose of creating "an artificial skin" and comprising a multitude of measuring electrodes and conductor elements intermeshed between one another and embedded in a material whose electrical conductivity depends on the extent to which it is compressed.
Other patent applications could be mentioned relating to various other apparatuses for the purpose of measuring the pressure to which a support is subjected under the effect of a load, for example, and based on capacitive circuits.
The present invention lies in this category of apparatuses, and more especially it lies in the category of apparatuses for detecting impacts and for recognizing such impacts over areas of a certain extent in order to trigger selective actions.
Most of the above-mentioned systems are based:
either on active measurement by emitting waves whose reflection is essentially an on/off type of phenomenon providing all-or-nothing detection; or if such methods are required to provide finer distinctions or quantitative measurements, then very complex and expensive signal-shaping and calculating circuits are required;
or else on passive measurements in which a magnitude is calculated, which magnitude can then be quantified in order to interpret its cause, and a qualitative analysis may be made thereof in order to select the desired action; however reliability is poor and adjustment must be performed on a permanent basis.
None of these systems can thus be used for detecting impacts, under any situation and environmental conditions, while simultaneously, and above all, recognizing such impacts without using apparatus which is complex or which requires adjustment on a permanent basis.
One of the problems encountered is to be able to detect and select, in particular over a given area, at least one type of impact as previously defined, and to detect that type of impact only, under all circumstances and regardless of the state of said surface at the moment of impact.